1. Field of the Invention
This invention relates to a method of forming a patterned silicone rubber layer on a surface of a workpiece. More particularly, the invention relates to a method of forming a patterned silicone rubber layer as a protective and/or passivation layer for a silicon wafer surface.
2. Description of the Prior Art
In the field of electronic devices, silicone rubber is often used as a junction coating resin (JCR) for protecting the electronic devices mounted on a printed circuit board. In such an application, the silicone rubber is usually coated onto the circuit board by a potting method as is the case with applying an epoxy resin.
On the other hand, it has been recently found that, by properly adjusting the viscosity, the silicone rubber can be applied on a surface of a workpiece by a spin coating method. The spin coating method is known as a method for forming a uniform layer of a fluid resin on a workpiece surface, and therefore used widely in wafer process to form a photoresist layer on a silicon wafer surface for the purpose of lithography. The applicability of the spin coating method to the silicone rubber opens up the way for utilizing this material in wafer process for the production of semiconductor devices.
As one possibility of applications in wafer process, the silicone rubber may be spin-coated on a silicon wafer to form a uniform layer which works as a protective and/or passivation layer. In such an application, the silicone rubber layer need have windows or openings at the locations of conductor bonding pads to enable wire bonding. Further, the silicone rubber layer should have a sufficient thickness to provide an intended protecting or passivating function.
Then, a question arises how the silicone rubber layer can be patterned to have windows or openings while maintaining a sufficient thickness thereof. In fact, the silicone rubber is known to be chemically stable, so that a wet etching method using a chemical etching liquid cannot be adopted for this purpose. A dry or physical etching method such as the reactive sputter etching (reactive ion etching) is the only possibility. However, no resist material is currently available which provides a high etching selectivity in the dry etching method with respect to the silicone rubber. Further, the dry etching method provides a low etching rate relative to the silicone rubber, so that it is very difficult to form openings in a silicone rubber layer having a large thickness of e.g. 5-10 micrometers.